Silicone rubber-esterified silica composition



United States Patent Hard Rubber Company, ration of Connecticut No Drawing. Application June 17, 1952, Serial N0. 294,066

8 Claims. (Cl. 260-37) This invention relates to the manufacture of silicone rubber and more particularly to a new silicone rubber product and a new method of making the same.

An object of the invention is to provide a silicone rubber product in which the silicone rubber polymer has better reinforcement than has been obtained heretofore, and which has better physical characteristics than heretofore.

Another object is to provide a simpler and better process of compounding and curing the rubber and of controlling the characteristics of the product, and to provide a process by which in a given case better and more certain results are accomplished in a relatively simple and convenient manner, with a saving of material.

A further object is to provide a silicone rubber product of which one component is a high-molecular-weight polysiloxane rubber which is essentially a dimethyl silicone polymer, another component being inorganic siliceous material in the form of very fine particles presenting a large surface area, said particles being separate and insulated from each other but intimately dispersed in the polysiloxane rubber and serving as the sole curative agent of the rubber compound.

Another purpose of the invention is to prevent the agglomeration of fine siliceous particles in the process of compounding the silicone rubber.

Another purpose of this invention is to cause the agglomeration by heating of the already thoroughly dispersed siliceous pigment particles.

Synthetic rubbers, to a greater degree than natural rubber, generally need pigment reinforcement to develop high tensile strength and/or abrasion resistance. The unreinforced silicone rubber possesses negligible strength, and any finely divided pigment will cause some degree of reinforcement. It is generally true that the smaller the diameter of the particles of the reinforcing powder, the greater will be the reinforcement value. Pigments such as described later on, having ultimate particles diameters of the order of 60 to 140 A., have exceptional reinforcing power in silicone rubbers.

Small particle size is not, however, the only criterion for good reinforcement of silicone rubber. For example, certain carbon blacks have particle diameters that go Kilbourne, to The Connecticut New Haven, Conn., a corpofrom 1000 A. down to as low as 200 A. These carbon blacks are quite effective for the reinforcement of natural rubber or GRS, but for some reason as yet unexplained, carbon blacks are not good reinforcing agents for silicone rubber.

It might be expected that finely divided silica would be a good reinforcing agent for silicone rubber due to the chemical similarity of the silica, which may be represented by the formula SiOz, and the polysiloxane, which may be represented by the formula ((CH8)2SlO)n. The CH3 group in the latter formula is sometimes replaced in part by other alkyl or aryl groups, but it is represented in this manner because in general the commercial silicone rubbers are derivatives of dimethyldichlorosilane. While not all forms of silica are found to be good reinforcing agents for silicone rubber, the expectation that V z silica-type pigments would be good reinforcing agents for silicone rubber has been realized in a number of cases.

An example of this is a pigment known as a silica aerogel,

and composed essentially of silica (SiOa). Such a pigment is sold by Monsanto Chemical Company under the name Santocel CS. Its method of preparation results in a high degree of porosity so that the surface area, including all of the pores of the pigment, is said to be in the neighborhood of 500 square meters per gram. The ultimate particles of Santocel CS, as examined with the electron microscope, have a diameter of from 50 A. to A. As used for the reinforcement of silicone rubber, the Santocel CS is in the form of aggregates of a size between 30,000 A. and 50,000 A., but, due to the high degree of porosity and large surface area, a high degree of reinforcement is provided.

It is well known that the rubber molecules in silicone rubber have a relatively low degree of afiinity for one another. This is evidenced by the low freezing point of the rubber, the low tensile strength and elongation of the unreinforced rubber, and in other ways. Up to now Santocel CS has been one of the better pigments for reinforcing silicone rubber, indicating a great degree of affinity of silica particles for the rubber particles, and vice versa.

Other commonly used reinforcing pigments for silicone rubber, such as zinc oxide and titanium dioxide, are composed'of smooth particles of average diameter of the order of 2,000 A. to 3,500 A. These do not show the degree of reinforcement obtained with Santocel CS3 Other pigments which have been used include hydrated aluminum oxide, which has an average particle diameter of 5,000 A., and calcium carbonate, which has an average particle diameter of 500 A.

When we speak of attractive forces between silica particles and rubber particles, We are in reality discussing the wetting of an inelastic solid, a silica particle, by a liquid. the silicone rubber. The forces of attraction between a liquid and a solid are related to the degree of wetting of the solid by the liquid. For example, if water is placed on a thoroughly cleaned sheet of glass, the water will spread out and completely cover the whole surface. In this case the liquid is said to wet the solid. The angle made at the edge of the liquid between the upper surface of the water and the glass is very nearly zero, implying complete wetting of a solid by the liquid. If, on the other hand, the glass has been exposed to even a minute amount of oil or dust in the atmosphere, it will be found that the water, instead of spreading on the glass, will remain in drops or globules at the edge of which the contact angle will be much greater than zero. A liquid which would be absolutely non-wetting would have a contact angle of It is known that a liqued which thoroughly wets a solid is attracted by the solid with strong forces. On the other hand, a liquid which does not wet a solid is relatively easily removed from it. Thus when we speak of strong attractive forces between silica particles and silicone rubber we are in reality saying that the silicone rubber behaves as a liquid and thoroughly wets the silica particles with a contact angle approaching zero. Silica, such as Santocel CS or the new silica pigments to be described below, are examples of pigments which are thoroughly wet by the silicone rubber, while carbon black may be considered an example of a pigment which is not well wet by silicone rubber. The strong forces of wetting are readily observable in compositions containing a viscous liquid silicone rubber polymer of dimethyl silanol, and fine silica pigments such as Santocel CS or the new silica pigments described herein. The mixed compounds, even before curing or cross-linking, are stifi gregated together in groups.

by nitrogen gas, for example.

and elastic because of -the large forces existing between the silica and the rubber.

In order to get a clearer picture of the magnitude of pigment i-p'aiticles', let 113 :im agine -'a "pigment i-particl'e -of"-2 centimeters diameter and "then "imagine that it is subdivitl'ed repeatedly in steps in which the diameter *of the particles is subdivided ten'tirnes in each step. Thus the initialradius is 1 centimeter, t-he radius after thefirst subdivision is 0.1-bentirneter, *etc. If this -is done eight times the-pigment particles will have the radii'shown incolumn 1 f the following table. 'In this calculation the total volume'of the pigment is-held constant. *In columnZ of the table the number of particles=of each size is calculated, and incol-umn 3 the total area possessed by-all the particles of each siZe is shown.

TABLET [Let diameter 'Z 'cn1.]

, Particle Number Dista ce "Radius m Spheres Total Area R 1 between Particles 2(R-r) cm. sq. cm. cm. 1 1 41r l. 2 4 1 10 41r 10 12 04 00001 10 41rX1Q 000012 000004 000001 41r 10 0000012 0000004 0000001 10 41rX10 00000012 00000004 Radius of sphere available for each particle. The enormous increase in area between the pigment and the surrounding rubber is'seen'from column 3. Even I when the forces per unitarea betweenpigme'nt and rubber are only moderate (poor wetting), there is necessarily a la'rge increase in reinforcement because of the large area involved. When the forces are large, as seems to be the case with silica, the effect-of the large area for extremely small particles is 'even more evident.

Let it now be further assumed that all of the pigment is enclosed within a given volume. It is convenient to assume 'an initial volume surrounding the pigment equal to that of a sphere of 1.2 centimeters radius. If this is done, it may be calculated, as shown in'column '5, that the average distance between pigment particles becomes smaller and smaller.

--In order to bring about 'an intimate dispersion of the p'i'g'r'nent -in'the silicone polymer, We prefer to employ a pigment in which the particle has a radius not greatly in excess of A., or 0.0000005 centimeter, which is. betw'een the particle sizes shown in lines 7 and 8 of the foregoing table. Thus the avera'ge distance between particles lies between 0.0000004and 0.00000004 centimeter, or between '40and 4-A.

If,however, as in the present case, the rubber medium in which these particles are to be dispersed is'a semiliquid silicone rubber, we have discovered that, since the -'partieles"are so fine and have a highly active surface, it is highly desirable to insulate the particles from each other -"s'o-'-a's to-pIeVentorinhibit the formation of agglomerates 'o'r chains of pigment particles. Ordinarily 'this'agglomeratio'n or c-hain formation could occur before or during the n'aiitin'g of the pigment with the rubber. To avoid this, '-we-us'e='a "pigment which bears an organic coating firmly attached to the pigment. A suitable pigment for these purposes is manufactured by E.- I.duPont'de Ne'mours '& 00., and is commercially available under the general designation GS Hydro- 'pho'bic Silica, and the more specific designation This is an amorphous silica with ultimate :particles'of silica which are non-porous and have a diam- Water in therange of m 120 A. These particles are ag- The aggregates are fairly open so that .pores between particles maybe penetrated The averagepore diam- Tlie aggregates show a combined cm: is at least 40 A.

butyl alcohol. this'period is twice the surface area including the pores greater than one square meter per gram, but preferably from 250 to 400 square meters per gram. On part of the surface of the aggregates of ultimately smoothsilicatparticles there is chemically combined a surface coating of OR-groups in which R is abiitylradieal. Thepreferr'ed structure of tlie OR groups is such that the carbon atom attached to oxygen is also attached teat-least one hydrogen, each 0R group having no more "than '18 carbon atoms. Groups or-this type are ceimnonly "referred to "as "es'ters'ils. It is preferred to have the R group a normally butyl radical. This pigment maybe prepared as shown "in the following example:

PREPARATTON 'o'r '"PIGMENT "EXAI\IPLE I Qne volume of a solution of 0.48 N sulfuric acid is added at a uniform rate,ov'er a period of 30 minutes, at a temperature of about 30 C., to three volumes of a solution of sodium silicate, containing 2 percent i0z and having a molar SiOzzNazO ratio of 3.36. The amount of sulfuric acid solution is adjustedso that it is equivalent to percent of the Naz O in the original sodiumsilicate. The pH during this process drops from 11.3 to about 9. Violent agitation is provided to insure complete and instantaneous mixing. The temperature during the entire reaction is maintained below 40 C. e-clear sol resulting from this process step and containing L-S percent SiOz, is called the heel.

he heel is heated to C. Solutions of sodium silicate and sulfuric acid are added simultaneously at a uniform rate over a .period of two hours. The sodium silicate solution contains 10 percent SiOz and has a molar SiOz zNazO ratio of 3.36. Enough 4 percent sulfuric acid solution (approximately equal in volume to the sodium silicate solution) is addedsothat 80 percent of the N220 in the silicate solution is neutralized during the addition step. The addition of silicate and acid is continued until one part of SiOz has been added for each part of SiOz present in the heel. During the additions the pH of the heel slowly rises from 9 to 10 and is then maintained at about 10. Vigorous agitation is employed so that the mixing is essentially instantaneous. A silica precipitateis thus obtained.

A 2 percent solution of a mixture of cetyl and lauryl trimethylammonium bromide, 0.16 percent of the mixed compounds is added, based on the weight of the silica. The slurry is filtered and the wet filter cake reslurried in water. The reslurry is adjusted to about pH 7 with dilute sulfuric acid, and then filtered and the filter cake washed tertiary butyl alcohol.

The filter cake which is wet with normal butyl alcohol is then slurried with an equal weight of anhydrous normal butyl alcohol and placed in a still with an 18 plate bubblecap column operated with a reflux ratio of 10: 1. Normal butyl alcohol is distilledslowly from the slurry over a period of 48 hours while maintaining the original volume of slurry in the still by the addition of anhydrous normal The volume of distillate removed during volume of the original slurry.

The finalproduct is then filtered from the slurry and dried.

Thisp'roducesa silica pigment in the form of an organowith 'a specific surface area of 250 to 400 M /g. and an 300F wAs a ;result, the{tensile strength was decreased V to 782;, the nat on o ,Pe t n e a a d ness hadincreasedtoGQ. h ws thatin tbis case; as shown by the lowered elongatiomfi lq fin ilg 'fl fim chemically? boundfto it -OR,groups fwh gin'ert solvent; anhydroiis ether. To theresulting mixture: 1 1 00, parts".(l.35 mols.)-of; nebutyl'aleohollwere added -OR groups wherein R is a hydrocarbon radical, whe the carbon? atom attached to'oxygen is also attachedt atj least one hydrogen, and where'each g oup hasno Electric SE-76 inthis, case) was placed on percent, and the Shore Hardness was 5 3 K ther cured by h ea'tingfor atotal p'eriod'of at butylxradicaL, p .4, PREPARAT ONR FPIGMEN'I E A P iAnothersuitableipigment was H ing procedure r. .1 Z 1 I v l parts (0 58 mols.) of silicontetrachlonde n 75 partsof anhydrous ether' wereladded 'SlQWlY, wi'tliyi'goroils stirring, toac'o'oled '.'C.) mixtur efconsisting of 15.85 parts (0.884 mo ls.) of water in.150fparts of an aridl mixture was heated, This,producedFa'pigment the"aggregates'ofwwhich were relatively coarse in com pa'risdn'ftofthe product of the ,next preceding ,example,

ere R-is a normal, 5:

but the internal structure of inorganic; siliceous material had a spec'ific surface area of 100 square meters per gram, and there were chemically bound tothe i ernal-struc fa'cture of rubber products in which the 'rubberfmixture or compoundcontains intimately mixed silicone polymer 7 and the fine-particle hydrophobic silica-in; case M EXAMPLE A v 100v volumes ofjdiniethyl silicone rubber K Gener n s eemi and 15 volumes of hydrophobic silica (dupont GS199S.)

compoundwas pressed ina mold under a. pressure of approximately 400' lbs'. per square inch, and heated until thetemperature in the mold reached 230, F.,:' a was thenheld at this'temperature for 15 minutes} As a result of this treatment, the tensile had "increased to 1660, the ultimate elongation had decreased to 9 l3 EXAMPLE 0 prepared as in Example A, but in this jcase. 2 pe'rcent of benzoyl peroxide, based *on the weight of" the rubber,

duPont GSIQQS -the following examples ar giyeng 111's glieating, the mold was cooled 'to 150 F; and opened. t The slab of partially cured rubber'was removed and Heated b in a circulating-air-oven for one hour-at 300 -1 The resulting product was found to have' al tensile strength of 1025'lbs. per square inch, an ultimate elongation of example in making a gasket designed to was an over-curing efiect; -I -Iowever, for. certain pui-ploses such an ,over-curedgcondition might be.

temperature of from 200 to 300" EXAMPLE E.

l 00 volumesot dimethyl silicone rubber *was' mixed I marketed by Johns-Manville Sales"Corporation-and-commonly employed in the manufacture of silicone rubber; The curing periods were as follows: Mold (press) cure, 15 minutes at 230 F.,fand"ovencure, 16 hours at 400 F.'

The" GS199S "was the only"curing. agentused. Tests [showed the following results: t

tea "125 volumes, Celite Superfloss ensi fst rength 845p. s, i., elongation 563 percent, and'ha'rdness 57. 4

(3)1fGS199S Silica 15 volumes, :Celite Siiperfloss: l0volurries, tensile strength 813 ps. .i., elongation 463 percennhaidness 6 1; v

b V EXAMPLE F",

- *Here 2 percent of benzoyl peroxide based onthe weight of the rubber was used as a curative in addition to the GS199S silica-other conditionsbeing as in Example- E.

5; volum'es; tensile-strength 900 pisl i.,elongationi225 f "(2) 5 681993 Silica 15 volumes, Celite Superfloss were thoroughly incorporated in the rubber. .;.The' mixed 40 l0"volu'mes, -tensile 910, elongation 2 00 percentghard ness 77.; r (3) ."With further increase =in ,the' volumes of Celite and. the hardness increased. f i

' -In*theseexamples the silicon e rubber was dimethyl silicone rubbermanufactured" by The General Electric Company underthe designation SE-.76. This particular rubber is polymerized with sodium hydroxide as a catalyst.

0, The molecular weight is approximately 1,000,000. This and heated, andthen given an'ovencure ofiqoneqhour i l at 300 F., as in 'Example A-g The resultingflproduct;was found to have a tensile strengthlof 1,008; :antelongationw of 525 percent, and-a Shore Hardness of 61. Comparing i A slab prepared; in 'furthese results with :thos'enof'Example A, it-will be seen 70 that this rubber was stiffer and harder; indicatinglhat ffrubber. was. formerly .known under the designation p The mixingwas accomplished in the usual manner on the customary open mill, but other customary milling procedures can'be used.

his to be noted that in Examples A and B the components'of the compound or mixture are the polymer and thehyd'rophobic silica, there being no special curing agent or vulcanizing agent such as commonly employed due to Another mixture. of silicone m rb Pigment was the fact that, when the said two components are brought.

into intimate mixture, the silica itself acts as a curative or'vulcanizer. V

In the process of bringing about an intimate mixture of the polymergand thepigmentparticles, the coating of n-butyl alcohol or equivalent coating carried by each pigforming-chainsio'r aggr in the rubber; 11min coating' has been dislodged or removedby the; rubber or otherwise in a heating or curin process.

desirable, f or function ata" 5 the tensile strength and elongation dropped f (3), 'I'n addition,- it increases the wetting of the silica particles'f by the silicone" rubber by increasing the forces ofattra'ctidnbetweenthe *siliconerubben'and theorganicting oirthe pigment. a 4)'-'fT-h e coating, being chemically combined-with the pigmenti 'c'onceals a'powerful vul'ca-nizi-ng capability untilit is removed by heating, which. causesthe coatingto vaporize and disappear.

The presence of the coatingmakes it possible to incorper-ate higher loading of pigment in silicone rubber than would: ordinarily bepossible,,thus,.increasing the strength andothercharacteristics. of the product Furthermore,,the coating,{whichis easily wet by the rubber, permitsthe ultimate, degree,- of. dispersion-of thepigment; in the. rubber..

, Although it is not as yet possible to. explain, exactly whataoccurs when the finehydrophobic. silicanwith its orgauophiliccoating is. incorporated in. silicone. rubber, az very unusual. 'resulthas been observed in that the,pigment causes vulcanization of the rubber even though. noconv e'ntional vulcanizing agent is added to. the. mixture.

The, degreeof strength enhancemenuattained by theuse another, as caused by the increasing pigment .concentration 3 possible; in". this procedure, and the more. active; the

surfaceslofthe: particles become as. -and =when thenco'ating is removed under the etiect of the heat employed. incuriirg;.-'the 'greater'is the-.degreeaofcure or;'.vulcanization. weibelievei thatjthe extraordinary reinforcement that is obtained byusing thesilica and the rubber alone; as components: of, the compound, ,is-accounted for in some measure by; first, the: extremelyhighldegrce: of dispersion of the pigment, second, the closeness of the-pigment; par. ticles: t0: eachother thirdythe extremely large area of theppigment rubben interface, 'and,; fourth,;:becausethe silica;particles, individually.- a'nd -en; masse,=are =well--wet by therubber;

ties i n all respects similar but superior to, those of'ordinary vul'canized silicone rubber.

-Ord-ina'ry vulcanizable compositions undergo a thermosetting operatio'n at'a certain critical temperature which depends upon the critical temperature at which the vul cani'zing ingredients undergo reaction first among themselves andv eventually with the elastomer, setting up crosslinlcspr chemical bonds between the molecules of the elastoniei'. In our compounds, vulcanization may be caused by physical forces occurring between silicone molecules and thesurfaces of the finely divided particles of silica, or it may be caused by some as yet unknown chemical reaction. Thecross-links may also be formed between the chemically active silica particles, after the coating has been removed, each particle being firmly ern-j bedded in the rubber by physical forces such asthos'e of wetting. In any case, these forces can be controlled by the specific surface characteristics of the silica particleswhich are subject to change at elevated temperatures. Thus, the rate of thermos'etting of the composition of the. rubber and herein described pigment may be controlled eitherby adjusting the proportion of surface active pigment or by controlling the degree of heating the mixture receives, or byboth. The degreeof heating refers to the 7 temperature employed and also the time to which the composition is exposed to the heat.

'Thehardness and strength of the product is also affected by the nature of the silicon rubber or silicone 'oils employed. By varying these conditions there can be obtained almost any desired combination of fluidity or plasticity before heating, with elasticity or rigidity after heat'treatment. For example, by replacing a part of thefrubber with a lowerlmolecular weight silicone oil. and adding a smaller portion of surface active pigment, a semi-fluid composition that will flow before it is heated is obtained. This composition can then be set toan" elastic rubbery solid by moderate heating. On the otherhand, if a rubbery polymer is employed and a relatively high proportion of the active pigment is incorporated therein and the resulting mixture is cured by heat, thereli-s obtained a stiff, flexible mica-like product that is' more or less hard' depending on the ultimate degree o'f heatihgttime and temperature) that removes the protective coating from the pigment. particles and exposesthe active surface to the, rubber.

In connection with Examples A andB, Table II, which follows; is. of interest as showing the superior reinforce- TABLE'II v Compounds omitting a curing agent i "V oLBigt Particle Time Temp. Tensile Hard- Compres- Pigment, Diameter" of'cure, 0! cures, Strength, Elong, ness sion set 1 l hours 1 F. p.s.i. Percent: ShoreA 22hrs.@

. 1st "1' 300 1,025 1,150 37 i I a at as, at as 1 15 2 1, :"G3 99 255- -140 1 300 1,245 1,000 62 97.2 as 24 300 1; 720 775 91. 0 25' 24 400 897 275 89 65,4 Silica Aerogel x 15' 1 1 I 300 No'cure (tensile below 100p s 1) ffsantoeeicsi-.- -20 50-150, .1-; 300 100, 100 33 20 .24 400 286 225 67 The explanation; .of -thisi phenomenon i ptl; exact ly ment obtained'inithis case with (381998 as a pigment known; It is; however,;obvious; that: twoithings-iarezhappeninga-m (1) with: ultimate particles of, diameters; of. apg proximately 100 7A. 'SllChvaS' exhibited bihfisantocel v( 38 OIithBul'iCW: silica pigments, disclosed3-herein', the; distances the iorder ofglw A.

betwecnparticles necessarily are of: or smaller, astshowniinfllable;hand

the interfacial without the-'useof a specific curing agent.

,Santocel ,CS is acommonlyrecognized form of silica --gel al'so "termed an Aerogelproduced by conventional arcatbet-wee'n:the:.pigments -and the irubbepbepomes ,ivery large; in; thistc'ase ofzhthe'; order of: 10 -"squareccentimeters per,.-gr..am; of ipigment 1 r 2 when this cure:omvulcaniaatiomoccurs, th oduct becomes insoluble in organic solvents, it exhibits qualii examples in the table the volume of the pigment was increased to 25. The unusually good results obtained with the GS199S are apparent from the figures given for tensile strength, elongation, hardness and compression set.

In this table the results obtained with the 681998 are compared with those obtained with one loading of volumes of Santocel CS and 2 loadings of volumes of Santocel CS, under the same conditions.

It is apparent from the table that, a respects the GSl99S pigment, when using a loading of 15 or volumes of the pigment per 100 of rubber, a satisfactory cure is obtained without the use of any vulcanizing agent, the 25 percent loading being superior to the 15. Under these same conditions, however, when the Santocel CS was used no cure of the silicone rubber wa obtained with a 15 percent loading, and only a very slight degree of cure obtained with the 20. The results shown demonstrate that, when the extremely fine hydrophobic silica is well dispersed in a silicone rubber mix and heated, it performs a function far beyond any reinforcing function that has been realized previously, because the surface character of the pigment enables it to be dispersed completely even when high proportions of the pigment are used, and because the silica performs a function in the nature of a vulcanizing function that has hitherto been unknown. This may be 'a physical vulcanization as distinct from the usual chemical vulcanization, or it may be a chemical combination of the active silica surface with the silicone molecules by some mechanism as yet unknown, but whatever the explanation the product has the superior properties shown by the table, and this is brought about by a simple operation of mixing and heat ing two components, making a third component (curing agent) unnecessary.

The procedure described makes it possible to cure silicone rubber thoroughly without producing low-stretch, resinous products. Long cures are desirable in silicone rubber compounding in order to stabilize the product and obtain low compression set properties. It is also possible to obtain a better control of the vulcanization. Moreover, the use of this process, as indicated by way of example in Examples A and B, and in Table II, makes it possible to eliminate the use of a hazardous material (benzoyl peroxide or similar material) from factories and storage rooms. Another advantage is that compounds produced in this manner do not contain residual benzoic acid or like acid which is a depolymerizing agent for the vulcanized silicone product. A further improvement arising from this procedure is that the vulcanizing action is not stopped or retarded by the presence in the mixture of suchsubstances as aluminum oxide, titanium dioxide, or. carbon black, which have been found to inhibit or even prevent the action of benzoyl peroxide as a curative for silicone rubber. When used in 'suflicient quantities, the hydrophobic silica will cause useful vulcanization to occur in the presence of these pigments, even though cure cannot be obtained in their presence by using benzoyl peroxide in normal amount. In fact, when sufficient pigment is present to contact a large proportion of the rubber moleculesl0 volumes per of rubber is the practical minimum-our experience shows that a cure is obtained regardless of the presence of other types of pigment in the mixture.

It has been noted that the pigment of Example II is one in which the aggregates are relatively coarse in comparison to the pigment of Example I. However, the Example II pigment, when ground and compounded with silicone rubbers by procedures similar to those of Examples A and B, brought about a satisfactory cure in the absence of any other curing agent.

With regard to Examples C and D, where a small percentage of benzoyl peroxide was used as a third component of the compound, it is noted that the best results are secured by making the addition of the curing agent only after the silica has been thoroughly incorporated in the rubber. In Example C the resulting product was stiffer and harder than in the case of Example A, indicating that the benzoyl peroxide enhanced the cure. On the other hand there was a decrease in tensile strength. In Example D there was produced an over-cured effect, although the material produced could be used for one or more purposes as indicated in the example.

Examples E and F show the use of a filling material which in this instance is diatomaceous earth, in a composition With the 681995 silica and the silicone rubber, and in the case of Example F benzoyl peroxide as an additional curative. These compositions are given by way of example only, and it is to be understood that numerous substances are suitable for use as filling material. Suitable filling materials will suggest themselves to those skilled in the art. It is indicated herein, however, that such pigments as aluminum oxide, carbon black, and titanium dioxide should not be used as fillers or pigments in compositions in which the benzoyl peroxide is employed as a curing agent, for the reason that these materials have been found to inhibit or prevent the action of benzoyl peroxide as a curative. While in the examples given in Tables II and III a single reinforcing agent was used, for the purpose of clarifying the effects in respect to physical characteristics, the use of additional substances having one or more functions in the compounds, where the additional material is inert or is entirely compatible with the other ingredients, is envisaged, and except as shown by the context there is no limitation to compositions containing the organophilic silica pigment only. Unless otherwise indicated in the context, the term filling material is used in a broad and general sense.

In connection with the employment of a curing agent, e. g., benzoyl peroxide, as an additive, reference is made to the following table showing the results secured in compounding diiferent batches of silicone rubber by a procedure similar to Examples C and D, using 100 parts of the silicone polymer, 2 parts of benzoyl peroxide based on TABLE III Compounds contammg a curing agent Vol. Pig. Average Hardloading Particle Time of Temp. of Tensile Elong., ness Pigment per 100 Diameter Heating, Heating, Strength, Percent Shore Vols. oi A hr. F. p. s. i A

Rubber Zinc Oxide 50 2,000 1 300 408 550 56 Titanium Dioxide 40 3, 500 1 300 587 408 46 Silica Aerogel Santocel CS... 20 l 50-l50 1 300 887 308 61 Hydrated Aluminum 0xide 40 5, 000 1 300 217 917 32 Calcium Carbonate 40 500 1 300 260 888 36 S 15 1 60-140 1 300 1, 008 525 61 GS199S"- 25 1 60140 1 300 1, 615 613 77 GS199S 35 1 60-140 1 300 1, 490 663 86 These are ultimate particle diameters as seen with the electron microscope. The ultimate particles are present as aggregates which may be as large as 50,000 A. in the case of Santocel CS or more than 1500 A. in the case of GS199S". Since the aggregates are porous, the ultimate particle diameter is the important figure.

theipolymenand different pigments as shown in the table.

The pigments "used were, zinc .oxide, titanium dioxide, silicazaerogel, hydrated aluminum oxide, calcium-.car-

.bonate, and GSl99S, and, as will be noted, the table shows the tensile strength, elongation, and hardness of the resulting-cured products.

It will be noted that three different tloadings of the GeneralElectric SE46. Theresults show very clearly the greatly improved physical properties of the product where the fine hydrophobic silica was used. The smallest loading of the 631998 -(15 volumes) gave a-tensile of 1008, an elongation of 525, and a Shore Hardness of 61. The second loading of 681998 (25 volumes) produced .-.a tensile of 1615, an elongation of'613, and a Shore Hard- ,ness of 77. It is hardly necessary to comment on the improvement over the comparative compounds, of which the silica aerogel was the best.

'In producing all of these compounds,-the time of heating was one hour and the temperature 300 F. With this heating period and temperature the surface characteristics of the pigment change only .to a slight degree. However, with further heating than is shown in this table, the molded products containing GS1'99S, which are apparently already cured with benzoyl peroxide, begin to show also vulcanization by the physical .or chemical forces of attraction of the fine particles of hydrophobic silica, as

a result of which'the products become stiffer, harder, and stronger. While it has been said above that the use of {benzoyl peroxide as an additive enhances the cure, in

comparison to a product containing only silica and silicone polymer, it may also be said that the fine hydrophobic ,silica enhances the cure obtained by the employment of benzoyl peroxide or like .curing agent in the compounding ofsilicone rubber.

The efiect of the further heating referred to in the 'next preceding paragraph is shown by the following .ex-

ample:

EXAMPLE G A recipe containing 25 volumes of GS199S silica pigment and 2 percent of benzoyl peroxide based on the weight of the dimethyl silicone rubber, had the following properties when cured inan oven for l hour at 300 F. (following press cure of 15 minutes at 230 F.): tensile strength 1615 p. s. i., elongation 613 percent, hardness 77; tensile strength at 200 percent elongation .419 pus. i. Thesame recipe, when cured for 24hours sile strength and elongation, as, in this example, the tensile-strength at 200 percentelongation. The resistance to :permanent'set after compression is another property that is generally improved by over-curing. For example,

by curingthe aboverecipe 24-hours-at 400 .F. (a higher temperature), the compression set was 49 percent as compared with 9415 percent for the original stock that was cured 1 hour -at 300 F. Other data on the 24-hour cure at 400 F; were: tensile strength 79 p.'s. i., elongation 75 percent, and, hardness 1 93.

Another polymer, General Electric SE-79" may -'be used in some ,casesalthongh results are not a satisfactory ber believed-to be er higher :molecular weight than .a notable improvement in In the curing of silicone rubbers it is usual to give the mixture preliminary heating in a mold of from 5 to 30 minutes at a temperature between 225 and 275 'F. This is followed by heating in an oven fora period from one to 48 hours at a temperature between 300 and For producing the results contemplated by the present invention, there should be a heating of ten minutes or longer at a temperature of 230 F. or higher. In preparing a mixture where volumesof siloxane polymer are used, the volume of pigment should be from '10 'to .130 parts. Wherebenzoyl peroxide is used as a curing additive, it should be in the amount of 2 percent or less based-on the weight of the polymer.

In place of benzoyl peroxide another member of the group consisting of benzoyl peroxide, diacyl peroxide, .acetyl benzoyl peroxide, diacetyl peroxide, and tertiary butyl perbenzoate, may be used, such curative being in quantity ,asrmentioned above. The curing agent used as an additive may be a suitable one commonly used in the manufacture of silicone rubber.

Ihejproducts of the invention have in many cases high tensile strength and high elongation, as shown by the preceding tables. The tensile strength with the-preferred .polymer is atleast 900p. s. i., and more often at least 1,000 p. s. i., and the elongation is at least 200 percent.

However, in the broader aspects of the invention we do tion but which might not result in physicals as shown above.

By the method described, polymeric siloxanes are rendered 'insolublein organic liquids selected from the class comprising alcohols, ketones, aldehydes, hydrocarbons, phenols, ethers, and halogenated derivatives of the ethers.

There is abetter reinforcement of the silicone rubber polymer than has been obtained heretofore, resulting in the physical characteristics of the product. It is obvious, also, that the invention results in a very notable improvement in this art whereby vulcanization is accomplished through the use of the reinforcing pigment which here has an additional new function obviating the need of the usual curatiive agent.

It is obvious also that the invention provides a novel method of compounding silicone rubber products and of controlling their physical characteristics.

Various changes may be made in the ingredients of the compounds and their proportions and in the detailed procedure without departure from the principles of the invention or the scope of the claims.

What we claim is:

1. The method of making vulcanized silicone rubber which comprises preparing an intimate mixture including a high-molecular-weight polysiloxane rubber gum which is essentially a nonelastic dimethyl silicone polymer, and an organophilic powder in a supercolloidal state of subdivision having an internal structure of inorganic amorphous ,silica, with a specific surface area of 250 to 4001M ./,g. and an average pore diameter of at least 4 -millimicrons, the internal structure having directly chemically bound to it -OR groups where R isanormal butyl .radicaL'the organophilic solid being present-in amounts rsufiicient to effect vulcanization of the rubber and being the sole curing agent, and heat- 13 ing said mixture for at least minutes at a temperature of at least 230 F. to form a vulcanized product.

2. The method of making vulcanized silicone rubber which comprises preparing an intimate mixture including a high-molecular-weight polysiloxane rubber gum which is essentially a nonelastic dimethyl silicone polymer, and an organophilic powder in a supercolloidal state of subdivision having an internal structure of inorganic amorphous silica, with a specific surface area of 250 to 400 M /g. and an average pore diameter of at least 4 millimicrons, the internal structure having directly chemically bound to it OR groups where R is a normal butyl radical, the organophilic supercolloidal solid being present in an amount of from 10 to 50 volumes per 100 volumes of silicone rubber and being the sole curing agent, and heating said mixture for at least 10 minutes at a temperature of at least 230 F. to form a vulcanized product.

3. The method of making vulcanized silicone rubber which comprises preparing an intimate mixture including a high-molecular-weight polysiloxaue rubber gum which is essentially a nonelastic dimethyl silicone polymer, and an organophilic powder in a supercolloidal state of subdivision having an internal structure of inorganic amorphous silica, With a specific surface area of 250 to 400 M /g. and an average pore diameter of at least 4 millimicrous, the internal structure having directly chemically bound to it OR groups where R is a normal butyl radical, the organophilic supercolloidal solid being present in an amount of from 10 to 35 volumes per 100 volumes of silicone rubber and being the sole curing agent, and heatinf said mixture for at least 10 minutes at a temperature of at least 230 F. to form a vulcanized product.

4. The method of making vulcanized silicone rubber which comprises preparing an intimate mixture including a high-molecular-weight polysiloxane rubber gum which is essentially a nonelastic dimethyl silicone polymer, and an organophilic powder in a supercolloidal state of subdivision having an internal structure of inorganic amorphous silica, with a specific surface area of 250 to 400 M /g. and an average pore diameter of at least 4 millimicrons, the internal structure having directly chemically bound to it OR groups where R is a normal butyl radical, the organophilic supercolloidal solid being present to efiect a vulcanization of the rubber and being the sole curing agent, heating the mixture in a mold for a period of from 5 to minutes at a temperatur of between 225 and 275 F., and then heating the product in an oven for a period of from 1 to 48 hours at a temperature between 300 and 480 F.

5. A vulcanized silicone rubber product prepared by the method of claim 1.

6. A vulcanized silicone rubber product prepared by the method of claim 2.

7. A vulcanized silicone rubber product prepared by the method of claim 3.

8. A vulcanized silicone rubber product prepared by the method of claim 4.

References Cited in the file of this patent UNITED STATES PATENTS 2,404,426 Bechtold et al. July 23, 1946 2,541,137 Warrick Feb. 13, 1951 2,563,555 Satford Aug. 7, 1951 2,610,167 Te Grotenhuis Sept. 9, 1952 2,657,149 Iler Oct. 27, 1953 OTHER REFERENCES Le Caoutchouc & La Gutta-Percha, November 15, 1938, page 315. 

1. THE METHOD OF MAKING VULCANIZED SILICONE RUBBER WHICH COMPRISES PREPARING AN INTIMATE MIXTURE INCLUDING A HIGH-MOLECULAR-WEIGHT POLYSILOXANE RUBBER GUM WHICH IS ESSENTIALLY A MONELASTIC DIMETHYL SILICONE POLYMER, AND AN ORGANOPHILIC POWDER IN A SUPERCOLLOIDAL STATE OF SUBDIVISION HAVING AN INTERNAL STRUCTURE OF INORGANIC AMORPHOUS SILICA, WITH A SPECIFIC SURFACE AREA OF 250 TO 400 M2/G, AND AN AVERAGE PORE DIAMETER OF AT LEAST 4 MILLIMICRONS, THE INTERNAL STRUCTURE HAVING DIRECTLY CHEMICALLY BOUND TO IT -OR GROUPS WHERE R IS NORMAL BUTYL RADICAL, THE ORGANOPHILIC SOLID BEING PRESENT IN AMOUNTS SUFFICIENT TO EFFECT VULCANIZATION OF THE RUBBER AND BEING THE SOLE CURING AGENT, AND HEATING SAID MIXTURE FOR AT LEAST 10 MINUTES AT A TEMPERATURE OF AT LEAST 230*F. TO FORM A VULCANIZED PRODUCT. 